1. Field of the Invention
The present invention relates to a gas sensor including a sensor element and measuring a predetermined gas component in a measurement gas.
2. Description of the Background Art
Conventionally, various measuring apparatuses have been used for recognizing a concentration of a desired gas component in a measurement gas. For example, as a device for measuring a NOx concentration in a measurement gas such as a combustion gas, known is a gas sensor having an electrochemical pumping cell structured by forming a Pt electrode and a Rh electrode on an oxygen-ion conductive solid electrolyte layer, such as a zirconia (ZrO2) layer. In this gas sensor, a diffusion control part communicating with the outside is formed so as to take a measurement gas from the outside into a sensor element and to apply a diffusion resistance suitable for measurement of a concentration of a desired gas component to the measurement gas.
For example, known is a gas sensor in which an opening communicating with the outside is provided in an end portion of a sensor element and a diffusion control part configured as a horizontally elongated slit is provided (for example, see Japanese Patent No. 3701124). Japanese Patent No. 3701124 discloses a gas sensor further including a gas inlet space before the slit-shaped diffusion control part mentioned above. Also known is a gas sensor including a diffusion control part formed by a porous body being filled in a portion of a sensor element for introducing a measurement gas from the outside so that a diffusion resistance suitable for measurement of a concentration of a desired gas component can be applied to the measurement gas during take-in of the measurement gas into the sensor element from the outside (for example, see Japanese Patent No. 3176890).
The gas sensors disclosed in Japanese Patent No. 3701124 and Japanese Patent No. 3176890 are, for example, installed in an exhaust pipe of an internal combustion engine such as an automobile engine, and used to measure a concentration of a desired gas component contained in an exhaust gas. In such an internal combustion engine, in a case where driving thereof is stopped, water vapor contained in the exhaust gas may be condensed to form a water droplet. Such a water droplet is not only accumulated in the exhaust pipe but also adheres to the gas sensor.
Generally, a gas sensor is mounted in an exhaust pipe so as to be substantially perpendicularly to a flow of an exhaust gas while a protective covering having a gas inlet hole through which a gas can freely pass is provided at the outer side of a sensor element. Therefore, in a case where a water droplet adheres to the inside of the protective covering, a part of the water droplet is discharged through the gas inlet hole. However, the water droplet still stays at a bottom portion of the protective covering. Since the bottom portion of the protective covering is in close contact with an end portion of the sensor element, when an internal combustion engine is re-driven, a considerable part of the water droplet staying at the bottom portion of the protective covering adheres to the end portion of the sensor element. Additionally, a water droplet staying within the exhaust pipe may splash within the protective covering to adhere to the end portion of the sensor element.
As a result, the adhering of the water droplet partially cools only the end portion, to cause a temperature gradient between the end portion and the inside of the sensor element. Moreover, a thermal stress resulting from this temperature gradient may cause cracking.
Additionally, water adhering to the end portion of the sensor element may reach an internal space through a slit-shaped diffusion control part formed in the end portion of the sensor element due to the capillarity. In a case where the internal combustion engine is driven in this state, the water rapidly vaporizes in the internal space to cause cracking in the sensor element.
In this manner, occurrence of cracking in the sensor element largely changes a diffusion resistance to be applied to the measurement gas, which is not preferable because a measurement accuracy is deteriorated.